1. Field of the Invention
This invention relates to improvements in an Albada optical finder.
2. Description of the Prior Art
In the general Albada optical finder according to the prior art, a mirror-surfaced field frame for a bright frame is provided toward the objective side on the eyepiece side, and a reflecting surface whose entire area half-transmits light therethrough or which was treated so that only the marginal portion thereof reflects light is provided toward the eyepiece side on the objective side, and the light rays from the field frame are reflected by the reflecting surface whereby the bright frame may be observed within the finder. FIG. 1 of the accompanying drawings is a plan view showing an example of the reflecting surface on the objective side used in such an Albada optical finder. The portion 10, indicated by hatching, is a reflecting portion for reflecting the light rays from the field frame and is formed of a surface having aluminum or the like evaporated thereon, and may be made into a complete reflecting surface or a half-mirror depending on the intended purpose thereof. The portion 20 inside the hatched portion is a transparent portion which takes in the light rays from the view field or the field frame illuminating light rays without loss. The above-described construction of the reflecting surface is such that in a case where the field frame seen in overlapped relationship with the view field, including the field frame for parallax correction, indicates only the outer peripheral portion of the photographing range, and where it is desired to have the field frame indicated also in the central portion of the view field, the reflecting portion 10 must be enlarged further inwardly, thereby making it sometimes impossible to secure the transparent portion 20. FIG. 2 of the accompanying drawings shows a variation in the reflection factor R (%) in the radial direction from the center of the reflecting surface shown in FIG. 1. A, B and C in FIG. 2 correspond to A, B and C in FIG. 1, A being the center of the reflecting surface, namely, the point of intersection between the reflecting surface and the optical axis of the finder optical system, B being the boundary between the transparent portion and the reflecting surface, and C being the end of the reflecting surface. In the reflecting surface of the prior art,
______________________________________ in the range from A to B (transparent portion), R = 0 in the range from B to C (reflecting surface), R = .gamma. (0 &lt; .gamma. .ltoreq. 100). ______________________________________
An optical finger device having such a reflecting surface has suffered from disadvantages which will hereinafter be described. That is, where the boundary B is coincident with the outer periphery of the finder view field, a blurred dark portion which is not clear is formed on the outer periphery of the view field, and where the boundary B lies inside the finder view field, a blurred dark portion is formed inside the view field, and on the outside thereof, a dark portion determined by the reflection factor of the reflecting surface 10 continues. The degree of blur of the boundary portion is prescribed by the difference in visibility to the eye between the virtual image of the boundary B by the eyepiece lens and the view field image. Accordingly, in a finder having an arrangement in which such difference in visibility is relatively small, the blurred image is perceived as a clear boundary by the eye, thus providing an obstacle to the view field observation within the finder. Where this boundary lies inside the view field, it has provided a more fatal defect.
In an Albada finder, the surface of the objective lens which is adjacent to the eyepiece is a surface passing therethrough the light rays from an object being photographed and also a surface reflecting the light rays from the field frame on the eyepiece side and therefore, it is difficult to clearly observe both the image of the object being photographed and the image of the field frame, namely, the bright frame, and the above-noted disadvantages have been regarded as being inevitable in the structure of the Albada finder.